Ed Miliband’s Labour party is in for a bruising at the general election in 2015, a survey suggests. Photograph: Lynne Cameron/PA Wire/Press Association Images

Labour is on course for a bloodbath in Scotland in 2015, according to a special Guardian/ICM online poll.

The Scottish National party, which took only 20% of the vote in the 2010 general election, has subsequently more than doubled its vote to reach a commanding 43% of the prospective poll next May. Scottish Labour, which secured a very strong 42% in Gordon Brown’s homeland last time around, has since tumbled by 16 points to just 26%.

The Conservatives sink from 2010’s 17% to 13%, while the great bulk of the 19% share that the Liberal Democrats scored last time around is wiped out as they fall by 13 points to 6%.

The SNP, meanwhile, would storm ahead from the mere six MPs it returned in 2010 to take a crushing majority of 45 of Scotland’s 59 constituencies. The Lib Dems, who currently hold 11 seats, would lose all but three, and the Tories would continue to languish with the single seat they currently hold.

Such dramatic Labour losses north of the border could easily offset the gains Ed Miliband hopes to make in England and Wales and potentially put Downing Street beyond his reach next year. But a unique analysis, conducted for the Guardian by Prof John Curtice, of Strathclyde University, suggests that the crude assumption of a uniform swing could actually be understating the catastrophe facing the party.

By breaking ICM’s data into four different categories of seat, Curtice reveals Labour’s decline is sharpest in those supposedly heartland seats where it previously trounced the SNP by more than 25 points.

Whereas Labour’s Scotland-wide vote drops by 16 points, it falls by 22 points in these constituencies while the SNP surges by 26. That combination is sufficient to wipe out majorities that were always assumed to be impregnable, and Scottish Labour’s Westminster caucus is left shrivelling to just three MPs.

“We are prospectively looking at the collapse of citadels that have always been Labour since the 1920s,” said Curtice. “That will seem incredible to some in England, but to those of us who paid close attention to Alex Salmond’s 2011 landslide at Holyrood, it would merely be the next chapter in the political transformation of a nation.”

He added: “It is becoming clear that the independence referendum has reset all the dials. Previously rock-solid Labour seats in Glasgow voted yes in the referendum, and this now appears to be giving rise to a particular surge of nationalist sentiment in those parts of Scotland where it was once assumed that the SNP couldn’t reach.”

With the nationalists also advancing by 20-plus points in the more competitive Liberal Democrat and Labour-held seats, they are on course to capture all the more obvious targets, securing a total of 53 seats under this more refined projection. The Lib Dems are again reduced to three and the Conservatives are wiped out entirely.

Curtice cautions that the polling samples are small for some categories of seat, but nonetheless believes there is enough evidence to conclude that the SNP is “on the march in heartland Labour seats, and – if anything – to a greater extent than elsewhere”.

ICM tested Scottish voters’ attitudes on a range of specific policy questions, which only confirmed the propitious mood for the new SNP leader and first minister, Nicola Sturgeon, as well as for her predecessor, Salmond, who is seeking to make a high profile return from Holyrood to Westminster in May.

Only 13% of voters worry that the proposals of the all-party Smith commission on devolution, which include full Scottish control of income tax rates and bands, go too far. Twenty-six per cent believe that Smith got the balance “about right”, whereas 30% believe the plans do not devolve enough.

One specific complaint of the nationalists about the plans is that London would retain control of corporation tax, something which Labour believes is necessary to avoid a cross-border race to the bottom in the rates paid by companies. But, by 53% to 23%, the voters are on the SNP’s side, saying that Scotland should be free to set its own corporation taxes.

Sturgeon stresses her opposition to Trident, the UK nuclear deterrent based at Faslane near Glasgow, and on balance Scotland’s voters are with her on this. Forty-three per cent want Trident scrapped and 37% want it retained.

ICM Unlimited interviewed an online sample of 1,004 Scottish adults aged 18 and over. Interviews were conducted across Scotland and the results have been weighted to all Scottish adults. They have also been weighted to respondents’ recall of the 2011 Holyrood election and to the independence referendum results. ICM is a member of the British Polling Council and abides by its rules.